Patent GB-787988-A (GB)
Title: Improvements in method and apparatus for reproducing magnetic signals by means of a cathode ray-tube
IPC: G11B11/115
Filed: 1954-04-12
Abstract: 787,988. Cathode ray tubes. LEVIN, S. April 12, 1954 [April 16, 1953], No. 10710/54. Class 39(1). Apparatus for converting signals recorded on a magnetic tape or similar members to electrical signals comprises a cathode ray tube in which electrons are modulated by the magnetic fields of the tape which permeate the target and extend into the tube and one or more collecting electrodes are provided for receiving the modulated beam and so far producing an output signal. The magnetic material may be solid permanently magnetized material or comminnuted particles of such material dispersed in a carrier, or dispersed in a binder and coated on a non-magnetic base, and the magnetic record member may be in the form of a tape, sheet, disc or cylinder. The information stored thereon may be complex signals, e.g., video. The electrons may be produced by cold, thermionic, photoelectric or secondary emission. The magnetic record member may serve as the target, Fig. 1 (not shown) the magnetic signal variably deflecting the beam from its initial position at the target edge or may be a separate member outside the tube end wall, Fig. 2 (not shown). In the latter case the target is formed by a member inside the tube or by a strip of conductive material sealed to a narrow slit in the tube end wall, the strip being scanned along its width. The target strip may be of non- magnetic material such as brass, copper or tin, or of magnetic material such as Ni, Fe or permalloy, especially an alloy of 78 per cent Nu, 22 per cent Fe which is heat treated at 1400‹C. in pure dry hydrogen with a magnetic field of 15 oersteds normal to its surface. Such a structure, it is stated, prevents the magnetic fields of the tape from spreading due to the creation of a preferred magnetic axes. The tape may be stationary or moving. In another form, Fig. 4, the electrons of the initial and return beams take different helical paths due to inherent transverse velocity components, and also due to the magnetic fields of the tape, and the beams as a whole have nodes and antinodes as shown, target 33 being at a node and collector 40- which may or may not incorporate a multiplier -being at an antinode. 38, 39 are accelerating electrodes. The scanning is (a) transverse to the direction of movement of the tape if tape 30 is moved continuously, (b) horizontal and vertical if tape 30 is fixed or moves intermittently, or (c) the beam is stationary but ribbon shaped and tape 30 moves. The incident beam may be (a) totally reflected (b) partially reflected or (c) may generate secondaries, at the target. In (b) some electrons are absorbed, some scattered and some reflected and the scattered and reflected electrons may be separated if desired. In (c) the high velocity primary beam produces low velocity secondaries, and a few reflected primaries which return over the same path as the incident beam and so are not collected by 40. In, (a), (b) and (c) the return beam spreads laterally in accordance with the magnetic fields of the tape so that a variable number of electrons are collected by 40. In another form, Fig. 7, a photocathode 47 is scanned by a beam 48 of light, infra-red rays, ultra violet rays or γ-rays and the low velocity photoelectrons are modulated by the tape 30. The photocathode may be constructed similarly to any of the forms of target of the preceding figures. 62 is a short or long magnetic lens, 61 an accelerating electrode and 57, 58 two collectors ; 57 preferably but not necessarily incorporates an electron multiplier. For a short lens 62 target 58 is in the equatorial mid plane of the lens or at the image plane (crossover) and for a long lens 62 target 58 is at an image plane. For no magnetic signal all the beam strikes 58 while magnetic modulation varies the point of crossover and causes a variable part of the beam to go to 57. Alternatively 57 may be apertured and in front of or in the same plane as 58. In a modification the beam is ribbon-shaped and is not scanned, but the tape moves to simulate scanning, Fig. 9 (not shown). In another modification, Fig. 10 (not shown) the target 51 is secondary emissive and is scanned by an auxiliary electron beam 50. In a further modification (not illustrated) of Fig. 7 members 57, 61, 62 are eliminated and target 58 made so large that the beam always strikes it, in which case the beam for magnetic signal strikes the edge of 58 and is deflected by the magnetic signal.